1. Field of the Invention
The present invention relates to a nonaqueous-electrolyte secondary battery incorporating a battery device accommodated in a case constituted by a laminated film, and more particularly to an improvement in a portion for heat-welding leads of terminals of electrodes and the case of the battery to each other.
2. Description of the Related Art
In recent year, a variety of portable electronic apparatuses including a camcorder, a portable telephone and a portable computer have been marketed. Under the foregoing circumstances, reduction in the size and weight of the foregoing electronic apparatuses has been attempted. As a portable power source for the electronic apparatus, a battery, in particular, a secondary battery, and more particularly, nonaqueous-electrolyte secondary battery (a so-called lithium ion battery) have energetically been researched and developed to realize a battery which has a small thickness and which can be folded.
As an electrolyte of the shape variable battery, research and development of solidified electrolytic solution have energetically been performed. In particular, a solid polymer electrolyte having a structure that lithium salt is dissolved in a gel electrolyte which is a solid electrolyte containing a plasticizer has attracted attention.
On the other hand, research and development have been performed about a variety of batteries of a type which is encapsulated in laminated films obtained by bonding plastic films or a plastic film and a metal member to each other to use the advantages of the battery of the foregoing type that the thickness and weight can be reduced. It is an important fact for a battery of the foregoing type to realize reliable sealing characteristic as well as or better than that of a metal tube.
To meet the foregoing requirement, an attempt has been suggested in, for example, Japanese Patent Laid-Open No. 56-71278. According to the disclosure, leads coated with resin are used to draw out terminals of electrodes from a sheet member which encapsulates a battery so as to improve sealing characteristics.
In Japanese Patent Laid-Open No. 3-62447, an attempt has been suggested to improve the sealing characteristics by using polyethylene denatured with acrylic acid or polypropylene denatured with acrylic acid as the resin employed to form the seal portion of the encapsulating member.
Another attempt has been suggested in, for example, Japanese Patent Laid-Open No. 9-288998. According to the disclosure, leads coated with polyolefine resin denatured with maleic acid are used to draw out terminals of electrodes from a sheet shape member which encapsulates a battery. Also the sealing portion of the sheet shape member for encapsulating the battery is made of polyolefine resin denatured with maleic acid. Thus, the sealing characteristic can be improved.
The method disclosed in Japanese Patent Laid-Open No. 56-71278 having the structure that the member for encapsulating the battery is made of one type of the resin, however, encounters inward penetration of water through the resin. What is worse, penetration and volatilization of the electrolytic solution cannot be prevented. Therefore, the foregoing method cannot preferably be applied to a battery which uses a solid electrolyte composed of organic solvent.
The methods disclosed in Japanese Patent Laid-Open No. 3-62447 and Japanese Patent Laid-Open No. 9-288998 has a structure that the overall polarity of the resin is raised to improve the sealing characteristics with the terminals of the electrodes made of metal. Therefore, the affinity with water is undesirably enhanced in spite of improvement in the sealing characteristics. As a result, there arises a problem in that water is undesirably introduced in the long term.